MM-54201 Refactor mention parsing in preparation for multi-word mentions (#25030)

* MM-54201 Move ExplicitMentions to its own file and rename it (#24932)

* MM-54201 Move ExplicitMentions to its own file and rename it

* Fix vet

* MM-54201 Refactor current mention parsing into MentionParserStandard (#24936)

* MM-54201 Refactor current mention parsing into MentionParserStandard

* Fix vet

* MM-54201 Unify user and group mention parsing logic (#24937)

* MM-54201 Add MentionKeywords type

* MM-54201 Move group mentions into MentionKeywords

* Fix flaky test caused by random iteration order

* Update server/channels/app/mention_results.go

Co-authored-by: Jesse Hallam <jesse.hallam@gmail.com>

* Address feedback

---------

Co-authored-by: Jesse Hallam <jesse.hallam@gmail.com>

---------

Co-authored-by: Jesse Hallam <jesse.hallam@gmail.com>

server/channels/app/mention_results.go

@@ -0,0 +1,91 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package app
+
+const (
+	// Different types of mentions ordered by their priority from lowest to highest
+
+	// A placeholder that should never be used in practice
+	NoMention MentionType = iota
+
+	// The post is in a GM
+	GMMention
+
+	// The post is in a thread that the user has commented on
+	ThreadMention
+
+	// The post is a comment on a thread started by the user
+	CommentMention
+
+	// The post contains an at-channel, at-all, or at-here
+	ChannelMention
+
+	// The post is a DM
+	DMMention
+
+	// The post contains an at-mention for the user
+	KeywordMention
+
+	// The post contains a group mention for the user
+	GroupMention
+)
+
+type MentionType int
+
+type MentionResults struct {
+	// Mentions maps the ID of each user that was mentioned to how they were mentioned.
+	Mentions map[string]MentionType
+
+	// GroupMentions maps the ID of each group that was mentioned to how it was mentioned.
+	GroupMentions map[string]MentionType
+
+	// OtherPotentialMentions contains a list of strings that looked like mentions, but didn't have
+	// a corresponding keyword.
+	OtherPotentialMentions []string
+
+	// HereMentioned is true if the message contained @here.
+	HereMentioned bool
+
+	// AllMentioned is true if the message contained @all.
+	AllMentioned bool
+
+	// ChannelMentioned is true if the message contained @channel.
+	ChannelMentioned bool
+}
+
+func (m *MentionResults) isUserMentioned(userID string) bool {
+	if _, ok := m.Mentions[userID]; ok {
+		return true
+	}
+
+	if _, ok := m.GroupMentions[userID]; ok {
+		return true
+	}
+
+	return m.HereMentioned || m.AllMentioned || m.ChannelMentioned
+}
+
+func (m *MentionResults) addMention(userID string, mentionType MentionType) {
+	if m.Mentions == nil {
+		m.Mentions = make(map[string]MentionType)
+	}
+
+	if currentType, ok := m.Mentions[userID]; ok && currentType >= mentionType {
+		return
+	}
+
+	m.Mentions[userID] = mentionType
+}
+
+func (m *MentionResults) removeMention(userID string) {
+	delete(m.Mentions, userID)
+}
+
+func (m *MentionResults) addGroupMention(groupID string) {
+	if m.GroupMentions == nil {
+		m.GroupMentions = make(map[string]MentionType)
+	}
+
+	m.GroupMentions[groupID] = GroupMention
+}